SN2 reaction of a sulfonate ester in the presence of alkyltriphenylphosphonium bromides and mixed cationic-cationic systems

Article abstract of DOI:10.1002/poc.1082

The effects of alkyltriphenylphosphonium bromides (C_nTPB, n = 10, 12, 14, 16) on the rates of S_N2 reactions of methyl 4-nitrobenzenesulfonate and bromide ion have been studied. Observed first-order rate constants are significantly higher than those found for other cationic surfactants for the same reaction. The results have been analyzed by the pseudophase model of micellar kinetics and show true micellar catalysis in the sense that second-order micellar rate constants are higher than the second-order rate constants in water, An attempt has also been made to investigate mixed cationic-cationic surfactant systems with respect to observed rates and pseudophase regression parameters. In addition, modeling of some cationic head groups has illustrated possible differences in head group charges and counterion interactions that may prove kinetically relevant. Copyright

Full text of DOI:10.1002/poc.1082

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2006; 19: 281–290
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.1082
SN2 reaction of a sulfonate ester in the presence of
alkyltriphenylphosphonium bromides and mixed
cationic-cationic systems
Michael M. Mohareb,1 Kallol K. Ghosh,2 Galina Orlova1 and Rama M. Palepu1
*
1Department of Chemistry, St. Francis Xavier University, Antigonish, NS B2G 2W5, Canada
2Visiting James Chair, Department of Chemistry, St. Francis Xavier University on leave from School of Studies in Chemistry, Pt. Ravishankar
Shukla University, Raipur, 492010 India
Received 16 January 2006; revised 10 March 2006; accepted 21 March 2006
ABSTRACT: The effects of alkyltriphenylphosphonium bromides (CnTPB, n ¼ 10, 12, 14, 16) on the rates of SN2
reactions of methyl 4-nitrobenzenesulfonate and bromide ion have been studied. Observed first-order rate constants
are significantly higher than those found for other cationic surfactants for the same reaction. The results have been
analyzed by the pseudophase model of micellar kinetics and show true micellar catalysis in the sense that second-order
micellar rate constants are higher than the second-order rate constants in water. An attempt has also been made to
investigate mixed cationic–cationic surfactant systems with respect to observed rates and pseudophase regression
parameters. In addition, modeling of some cationic head groups has illustrated possible differences in head group
charges and counterion interactions that may prove kinetically relevant. Copyright # 2006 John Wiley & Sons, Ltd.
KEYWORDS: SN2 reaction; kinetic parameters; micellar catalysis; pseudophase model alkyl phosphonium surfactants;
mixed micelles
INTRODUCTION
limited, and there appear to be no systematic studies of
this series with respect to its effects on the kinetics of SN2
reactions. One report has considered the use of
derivatized hexadecyltriphenylphosphonium surfactants
in an ester cleavage reaction,17 highlighting a further use
of these surfactants in their ability to be functionalized to
different degrees.
Mixed micelles have recently expanded into a wide
area of research,18,19 and the body of knowledge on
effects of mixed surfactant systems on rates of
hydrolysis,4,20–23 nucleophilic substitution,19,24,25 and
other reactions,26 continues to grow. Very few studies
have been carried out on the effects of cationic–cationic
mixed surfactants on the enhancement of rate constants.
In the present work, we have cursorily studied three such
systems: C14TPB and hexadecyltrimethylammonium
(CTAB) bromides, C14TPB and hexadecyldiethylethano-
lammonium bromide (C16DEEA), and C16TPB and
C10TPB. Molecular modeling technique of selective
head groups interacting with bromide ion was carried out
in the present study, to evaluate structures and charge
distribution that may be kinetically relevant.27,28
The use of novel surfactants in assisting a variety of
organic reactions is highly promising for basic and
applied research.1–5 The application of quantitative
analysis of kinetic data to micellar mediated reactions
and the implications of different models describing such
reactions are also topics of continuing interest.6–11
The majority of fundamental studies on micellar
mediated SN2 reactions have been conducted in the
presence of cationic micelles of quaternary ammonium
surfactants such as those with trialkylammonium, pyridi-
nium, and quinuclidinium bromides and chlorides.12–14
In the present investigation, the kinetics of the reaction
of methyl 4-nitrobenzenesulfonate (MNBS) with bromide
ion (Scheme 1) in alkyltriphenylphosphonium bromide
(CnTPB) surfactants (Scheme 2) has been studied.
These surfactants offer head groups that are signifi-
cantly bulkier than those which have been investigated
previously and also possess aromatic moieties at the
micellar surface. Recently, the self-aggregation properties
and thermodynamics of micellization for this series of
surfactants and their binary mixtures have been studied by
our group.15,16 Surprisingly, however, the body of
chemical investigations on CnTPB surfactants is rather
RESULTS AND DISCUSSION
*Correspondence to: R. M. Palepu, Department of Chemistry,
St. Francis Xavier University, Antigonish, NS B2G 2W5, Canada.
E-mail: rpalepu@stfx.ca
The reaction of MNBS and bromide ion was investigated
in the presence of CnTPB surfactants for chain lengths
Copyright # 2006 John Wiley & Sons, Ltd.
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282
M. M. MOHAREB ET AL.
−
−
O2N
SO3
CH3Br
O2N
SO3
CH3
+
Br
+
Scheme 1
C10, C12, C14, and C16. The range of concentrations over
which these surfactants were studied was limited some-
what by their solubility in water. Rate-surfactant profiles
were determined for each surfactant at 298.2 K and are
shown in Fig. 1 with theoretical regression fits (discussed
later). As in the case with other quaternary nitrogen based
cationic surfactant systems (TABS), an increase in rate
enhancement with increased surfactant concentration is
observed. In addition, for a given head group, this
enhancement is pronounced with an increase in hydro-
phobic chain length of the surfactant. This has been
observed previously for all other cationic systems for
similar anionic bimolecular reactions. The distinguishing
characteristic in these rate-surfactant profiles, however, is
the magnitude of increase in the pseudo first-order rate
constant (kobs) values as compared to other quaternary
ammonium bromide surfactants of similar chain
length.29–33 The values of the second order rater constant
(103 Km2 /Vm) to other quaternary ammonium bromide
surfactants of similar chain length31 are presented in
Table 1.
Quantitative treatment of the data
Each rate-surfactant profile was fitted to a regression
according to the pseudophase model (PPM). For a
second-order reaction between a substrate (S) and a
nucleophile (X), the reaction can be assumed to occur in
either the micellar or aqueous pseudophases so that the
overall rate of reaction is the sum of the rates in each
pseudophase and is given by the following equation.30,38
ꢀd½Stotꢁ
¼ kobs½Stotꢁ ¼ k2w½Swꢁ½Xwꢁ þ k2m½SmꢁXm
(1)
dt
In Eqn (1), km2 and k2m refer to the second-order rate
constants of the reaction, [Sw] and [Sm] refer to substrate
concentrations, and [Xw] and Xm refer to nucleophile
concentrations in the aqueous (w) and micellar (m)
pseudophases, respectively. For the nucleophile concen-
trations, [Xw] is the solution concentration of nucleophile
in the bulk water while Xm, which corresponds to the
interfacial concentration of nucleophile in the micellar
pseudophase, is given by:
A rate-surfactant profile at 298.2 K (Fig. 2) was also
determined for the surfactant, hexadecyltributylpho-
sphonium bromide (C16TBuB), in order to investigate
the extent of rate increase due to its bulky alkyl head
group and to compare this with the aryl substituted head
group.34–36 The observed rates were significantly lower
than those of C16TPB, though still higher than the
corresponding quaternary ammonium bromide surfac-
tants. Previous work.13,17 with trialkylammonium head
groups and SN2 reactions of similar substrates shows a
modest increase in the observed rate for hexadecyltribu-
tylammonium over trimethyl, triethyl, and tripropyl head
groups. If the case was similar for quaternary phos-
phonium head groups (i.e., that a hexadecyltrimethylpho-
sphonium bromide surfactant would only show a slightly
lower observed rate), it would indicate greater rate
enhancement for the phosphorous center than for nitrogen
centers.
½Xmꢁ
Xm ¼
(2)
Vm½Dnꢁ
where Vm is the molar volume of the interfacial reaction
region and [Xm] is the solution concentration of the
nucleophile in the micellar pseudophase. The value of Vm
is usually assumed to be in the range of
0.14 Mꢀ1 < Vm < 0.37 Mꢀ1, according to Stigter’s model
of micellar structure.29,30 Solving for kobs in Eqn (1),
substituting Xm according to Eqn (2), and incorporating
an equilibrium substrate binding constant (KS) yields the
following overall relationship:
ꢀ
ꢁ
k2m
Â
Ã
Â
Ã
k2w Brwꢀ
þ
Brmꢀ Ks
Vm
kobs
¼
(3)
1 þ KS½Dnꢁ
where the nucleophile (X) for these investigations has
been identified as bromide ion. This relates kobs to
changing concentrations of surfactant and bromide ion.
The theoretical lines in Figs. 1 and 2 are regression fits of
the data according to Eqn (3).
Most commonly for bimolecular reactions in ionic
surfactant systems, such as in this investigation, the
concentration of micellized counterion has been assumed
to follow a Langmuir form equation as given below:
R P+
Br
½Brmꢁ
KBr
¼
(4)
½Brwꢁð½Dnꢁ ꢀ ½BrmꢁÞ
In Eqn (4), KBr is the equilibrium constant that
describes binding of bromide ion to the cationic micelle,
R = C10, C12, C14 and C16
Scheme 2
Copyright # 2006 John Wiley & Sons, Ltd.
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SN2 REACTION OF A SULFONATE ESTER
283
20.0
16.0
12.0
8.0
C16TPB
C14TPB
C12TPB
C10TPB
4.0
0.0
0.000
0.010
0.020
0.030
0.040
0.050
[surfactant] / mol L-1
Figure 1. Influence of CnTPB concentrations on the observed rate constant for the reaction of MNBS þ Brꢀ at 298.2 K. Solid
lines are theoretical regression fits according to equations described in the text
[Brm] and [Brw] are concentrations of bromide ion in the
micellar pseudophase and bulk water phase, respectively,
and [Dn] is the solution concentration of surfactant present
in micelles. The value of [Dn] is equal to the difference
between the total surfactant concentration ([Dtot]) and the
concentration of monomeric surfactant, with the latter
usually assumed to be equal to the cmc.39–44
equation at each concentration of surfactant investigated
for a given bromide binding constant (KBr).
A wide range of values for the bromide binding
constant (KBr) were employed in the regressions in order
to examine the variance of the rate constant (km2 /Vm) and
substrate binding constant (KS) values. Conductometric
studies have shown that counterion binding in triphenyl-
phosphonium bromides is lower (0.13 for C10, 0.40 for
C12, 0.44 for C14 and 0.31 for C16 TPBs respectively) than
that found for other cationic micelles (i.e., quaternary
ammonium bromides).15 This has been attributed to a
decrease in effective micellar surface charge density in
TPB head groups (as is the case with hexadecylpyr-
idinium bromide) as compared to other non-aromatic
head groups, and this may be due to the significantly
lower aggregation numbers for CnTPB micelles that are
afforded by a head group of such bulkiness. Aggregation
numbers as determined by fluorescence quenching for
CTAB and C16TPB are 61 and 15, respectively.15
The focus in the present study was not so much on
bromide binding in these surfactants as it was an attempt
to investigate the nature of the reaction of MNBS and Brꢀ
and the binding of the substrate to the surface of TPB
micelles. Therefore, results of the PPM regression fits
have been presented in Table 1 in terms of the range of
bromide binding constants (KBr) used in order to examine
km2 /Vm and KS values. A value of 4.5 ꢂ 10ꢀ4 Mꢀ1 sꢀ1 at
298.2 K was used for the second-order rate constant in
water (kw2 ).33,37 The regression parameters for C16TBuB
are similarly included in Table 1. In all cases, the
regression correlation coefficients (r2) were greater than
0.97.
Dn ¼ ½Dtotꢁ ꢀ cmc
(5)
By taking into account the mass and charge balance,
Eqn (4) (the Langmuir equation that describes bromide
binding) can be transformed to the following quadratic
form30:
Â
ꢀ Ã
2
KBr½Brmꢀꢁ ꢀ ðKBr½Dnꢁ þ KBr Brtot þ 1Þ½Brmꢀꢁ
(6)
þ KBr½Dnꢁ½Brtꢀotꢁ ¼ 0
where the values of bromide ion concentrations in each
pseudophase can be found by solving the quadratic
Table 1. PPM regression parameters for CnTPB and
C16TBuB surfactants at 298.2 K
KBr Mꢀ1
103km2 /Vm sꢀ1
KS Mꢀ1
C16TPB
C14TPB
C12TPB
C10TPB
C16TbuB
100 (1000)ꢃ
2000
100 (1000)
2000
100 (1000)
2000
100
40.2 (5.87)
38.4
35.3 (4.13)
38.4
21.1 (9.92)
19.7
11.5
200 (76)
49
109 (71)
30
126 (39)
42
61 (39)
32
273
2000
100
2000
10.3
17.5
15.6
As with results for similar reactions studied in cationic
micelles,12 the decrease in the observed rate with a
decrease in hydrophobic chain length for the same
surfactant head group is reflected in the PPM regression
63
ꢃ Bracketed terms are the corresponding values for alkyl trimethyl
ammonium bromides (Ref. 31)
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284
M. M. MOHAREB ET AL.
18.0
16.0
14.0
12.0
10.0
8.0
C16TBuB
C16TPB
6.0
4.0
2.0
0.0
0.000
0.010
0.020
0.030
0.040
[surfactant] / mol L-1
Figure 2. Influence of C16TBuB concentration on the observed rate constant for the reaction of MNBS þ Brꢀ at 298.2 K. The
rate-surfactant profile for C16TPB is shown for comparison. Solid lines are theoretical regression fits according to equations
described in the text
parameters primarily in a decrease in the substrate
binding constant (KS), assuming constant KBr values with
varying chain length. This may be due to decreased room
for accommodation of the substrate at the micellar
surface, and therefore would be a reflection of the
decrease in micellar size with a decrease in the
hydrophobic chain length.
corresponds to such a bulky head group. In this case, the
mechanism of rate enhancement in these surfactants
would simply be an extension of what is believed to be
observed with the trialkylammonium surfactant series13
and would therefore also be observed with C16TBuB. The
difference in km2 /Vm values between C16TPB and
C16TBuB would then be due to the greater head group
bulk of the former. The effect would correspond to a
general decrease in the polarity of the head group region
that would therefore increase the rate of an SN2
reaction.37 Another possible explanation would be the
steric effects by the phenyl rings in the head group region
that would activate the bound MNBS substrate towards
substitution.
One interesting point to note from the kinetic
parameters is that KS values are nearly comparable to
those found for quaternary ammonium bromides while
km2 /Vm values are 3–8 times higher for phosphonium
bromide surfactants than their quaternary ammonium
surfactants. Although there is considerable deviation in
the values for substrate binding with a change in the
bromide binding constant, micellar rate constant values
differ only slightly (well within an estimated experimen-
tal error of 15%) with a wide variation in counterion
binding. The increase in km2 /Vm values would be even
more pronounced for strict second-order rate constants
(km2 ) if one were to consider that the molar volume of the
interfacial region of TPB surfactants is much greater than
that for quaternary ammonium head groups. It is likely
that the enhancement of the second-order rate constants as
a function of surfactant concentration can be attributed to
increased micellar and effective bromide ion concen-
trations at the micellar surface.
The parameters obtained for C16TBuB also a show an
enhancement in the second-order rate constant in the
micellar pseudophase over water and other surfactants
(depending on precise values for Vm) as opposed to a
simply greater concentration of the reactants at the
interfacial region. However, the extent of this enhance-
ment is not as high as for the TPB series.
Possible sources for the rate enhancement at the TPB
micellar interface may include an increase in disruption
of the hydration of the attacking bromide ion that
Mixed micellar systems
The rates of reaction of MNBS þ Brꢀ were also
determined for mixed cationic–cationic systems of
C14TPB-CTAB and C14TPB-C16DEEA at varying con-
centrations and compositions in an attempt to correlate
the observed rates and determined parameters with
system properties. Rate-surfactant profiles (Fig. 3) were
found for both systems at varying solution mole fraction
of C14TPB (aC14 ) and at constant total surfactant
concentration (25 mM, respectively). Both plots exhibit
curves that negatively deviate from a straight line of
ideality. In order to confirm that a straight line
relationship between the observed rate constant (kobs
)
and the solution mole fraction exists for an ideal system,
rate experiments were also carried out for the mixed
C16TPB-C10TPB system showing a linear relationship of
rate constant with mole fraction; the profile for this
system is shown in Fig. 4. This system has been shown to
behave ideally with respect to other micellar properties16
Copyright # 2006 John Wiley & Sons, Ltd.
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SN2 REACTION OF A SULFONATE ESTER
285
A
B
14.0
12.0
10.0
8.0
14.0
Experimental
Ideal
Experimental
Ideal
12.0
10.0
8.0
6.0
6.0
4.0
4.0
2.0
2.0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
Solution Mole Fraction of C14TPB
(C14)
0.4
0.6
0.8
1.0
Solution Mole Fraction of C14TPB
(C14)
α
α
Figure 3. Influence of solution composition (solution mole fraction of C14TPB) on the observed rate constant for the reaction of
MNBS þ Brꢀ for A. C14TPB-C16DEEA and B. C14TPB-CTAB mixed surfactant systems at 298.2 K with total surfactant
concentrations of 25 mM
(e.g., cmc, counterion binding, enthalpy of micellization,
etc.), so it is not surprising that it also behaves ideally
from a kinetic viewpoint. One would certainly expect that
for a reaction such as this, (which is believed to occur
preferentially at the micellar surface6), therefore a system
employing surfactants with the same head group would
offer an approximate picture of ideality. It is interesting to
note that for the inhibition of both this and similar SN2
reactions in the presence of cationic micelles by nonionic
surfactant additives, a comparable straight-line relation-
ship also exists between the observed rate constant and
the mole fraction of the additive.19.
In this light, the observed results for C14TPB-CTAB
and C14TPB-C16DEEA systems indicate that the mixed
micelles that are formed result in a lower observed rate of
reaction than would occur if the surfactants were
independent of each other. A possible explanation for
the observed results may lie, at least partly, in the
relationship between the compositions of surfactants in
the micelle with their overall solution compositions.
According to Rubingh’s approach, deviation from ideal
behavior is due to differing compositions of surfactant
monomers as compared to the bulk solution.45 From
conductivity studies and the use of the regular solution
approximation, the mole fraction of a surfactant in the
mixed micelle can be determined for its particular
solution concentration.46 In the case of C14TPB-
C16DEEA, for example, a conductivity study was
performed previously by our group,47 and the resulting
micellar composition plot is presented here in Fig. 5. The
straight line relationship represents the situation where
the mixed micelle composition is equal to the compo-
sition in the bulk solution, that is, x(C14) ¼ a(C14) in the
ideal case. According to this plot, there is more C14TPB
16.0
Experimental
Ideal
14.0
1.0
0.8
0.6
0.4
12.0
10.0
8.0
6.0
4.0
Experimental
Ideal
0.2
0.0
2.0
0.0
0.2
Solution Mole Fraction of C16TPB
(C16)
0.4
0.6
0.8
1.0
0.0
0.2
Solution Mole Fraction of C14TPB
(C14)
0.4
0.6
0.8
1.0
α
Figure 4. Influence of solution composition (solution mole
fraction of C14TPB) on the observed rate constant for the
reaction of MNBS þ Brꢀ for C16TPB-C10TPB mixed surfactant
system at 298.2 K with a total surfactant concentration of
20 mM
α
Figure 5. Mixed micelle composition plot as determined by
conductivity using Rubingh’s model for the C14TPB-C16DEEA
system at 298.2 K
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 281–290

286
M. M. MOHAREB ET AL.
16
14
12
10
8
C14TPB
A
C16DEEA
Mixed System
α(C14) = 0.9
α
(C14) = 0.8
α(C14) = 0.6
α(C14) = 0.2
6
4
2
0
0
0.01
0.02
0.03
0.04
-1
[surfactant] / mol L
16
14
12
10
8
C14TPB
CTAB
B
Mixed System
(C14) = 0.8
α
α(C14) = 0.6
α(C14) = 0.4
6
4
2
0
0
0.01
0.02
0.03
0.04
[surfactant] / mol L-1
Figure 6. Rate-surfactant profiles for A. C14TPB-C16DEEA and B. C14TPB-CTAB mixed surfactant systems at 298.2 K and
varying total surfactant concentrations for select compositions. Solid lines are theoretical regression fits according to equations
described in the text using a sample value of KBr ¼ 450 Mꢀ1
than is predicted by the solution concentration (a(C14)) at
low amounts of C14TPB until a mole fraction of about 0.6
after which there is a greater amount of C16DEEA in the
mixed micelles than given according to its solution
concentration. Therefore, the region of the largest
negative deviation from ideality of the observed rate
occurs at higher amounts of C14TPB in the solution
composition which corresponds to less C14TPB in the
micelles. This would suggest that with more C16DEEA
(the surfactant whose kobs value in the pure micelle is
much lower), there is a greater deviation from ideality in
the rate constant. Also, additional C14TPB (as the mole
fraction is increased to unity) is more effective in
increasing the observed rate. As for the source of the
negative deviation in general across the whole compo-
sition range, this remains unclear.
Rates of MNBS þ Brꢀ reaction were measured at
several concentrations for select compositions for both
C14TPB-CTAB and C14TPB-C16DEEA systems. The
resulting rate-surfactant profiles are presented in Fig. 6
with theoretical lines representing regression fits accord-
ing to Eqn (3), and the determined parameters are shown
in Table 2. Again, a range of bromide binding values were
used in order to confirm that km2 /Vm values do not vary
significantly, and for constant KBr values, the relationship
of km2 /Vm with solution composition proved to be similar
in nature to that of kobs. That is, there was a negative
deviation from ideality for km2 /Vm values for both mixed
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 281–290

SN2 REACTION OF A SULFONATE ESTER
287
Table 2. PPM regression parameters for select compo-
sitions of mixed C14TPB-C16DEEA and C14TPB-CTAB mixed
surfactant systems at 298.2 K
charges as implemented in Gaussian03. Figure 7 shows
optimized structures along with Mulliken and NPA
charges summed to the heavy atoms for the following
head groups: trimethylammonium (TMA), dimethylethano-
lammonium (DMEA), diethylethanolammonium (DEEA),
trimethylphosphonium (TMP), triphenylphosphonium
(TPP), and tributylphosphonium (TBP) cations.
According to Mulliken and NPA analyses, the positive
charge of the quaternary ammonium head groups (TMA,
DMEA, and DEEA) is distributed over the substituent
groups (essentially on hydrogen atoms); nitrogen atom
adopts a negative charge. In contrast, for the quaternary
phosphonium cations, the phosphorus center bears highly
positive charge. The NPAvalues are of þ1.59, þ1.73, and
þ 1.74 for TMP, TPP, and TBP, respectively. Thus, the
total charge of the substituent groups is slightly negative
albeit hydrogen atoms are positively charged (values are
not shown). Although it is not a scope of present
theoretical study, one might suppose that the difference in
charge distribution may cause the difference in the
structure of solvent shells that in turn may result in
different behavior of nitrogen- and phosphorus-contain-
ing head groups in counterion binding and/or the kinetics
of SN2 reactions in condensed phase.
C14TPB-C16DEEA aC KBr Mꢀ1 103k2m/Vm sꢀ1 KS Mꢀ1
14
0
100
2000
100
2000
100
2000
100
2000
100
2000
100
2000
100
2000
100
2000
100
6.17
5.11
8.23
7.12
536
96
311
70
470
93
394
84
257
65
109
30
233
64
537
99
0.2
0.6
0.8
0.9
1
12.6
10.3
16.9
14.1
21.1
18.4
35.3
38.4
C14TPB-CTAB
0
6.77
5.85
8.59
6.97
0.4
0.6
0.8
1
12.8
474
99
287
66
109
30
2000
100
2000
100
2000
10.2
20.0
17.6
35.3
38.4
The highly positive charge on phosphorus atom allows
suggesting that it may coordinate directly with the
bromide counterion. This suggestion however was not
supported by our computational results shown in Fig. 8.
The binding of bromide anion on phosphorus center of
TMP leads to the formation of Br-TMP, with Br
interacting with three hydrogen atoms of three methyl
groups. The TMA and TBP head groups form similar
systems. Comparison of Br-TMA and Br-TMP reveals
only minor differences in geometries and charge
distribution. For Br-TMP, bromide interacts with hydro-
gen atoms at somewhat longer distances and bears
slightly less negative charge. The TPP head group
interacts with bromide differently. Due to steric effect of
bulky phenyl groups, Br interacts with only two of three
ortho phenyl hydrogens with the third phenyl ring
oriented away from the Br. This may suggest that the
improved performance of TPP as a catalyst for SN2 attack
may be due to steric effects of phenyl substituents.
However it is not possible to point out whether the
increased activity is due to the charge on the phosphorus
atom or p–p interactions between the phenyl groups and
the substrate or to the steric effects due to the phenyl
moieties in the head group region. Further investigations
in this direction may throw light on this matter.
systems. Because Km values depend more strongly on
bromide binding, no relationship with system compo-
sition could be found for this parameter.
Molecular modeling
Molecular modeling along with ab initio study of selected
head groups interacting with bromide counterion were
performed in order to elucidate structures and charge
distribution which might be kinetically relevant. The
alkyl side chains of all the surfactants were simplified to
ethyl groups as the effect of added methylene groups on
electronic structure becomes negligible.27 In light of low
aggregation number of 15 reported earlier15 for C16TPB
and the low value of 25 for C16TBuB determined by our
group using steady state fluorescence quenching,48 the
examination at molecular level is relevant. Simple
modeling of a micelle of C16TPB using GaussView03
visualization program49(structure is not reported due to
˚
complexity) depicted a significant distance (over 14 A)
between surfactant head groups. Therefore, the compu-
tational analysis of interaction of a surfactant monomer
head group with counter ion was carried out in the gas
phase as a crude model of catalysis at the micellar
interface.
All computations were performed using Gaussian0350
program suite. Hartree-Fock (HF) ab initio method with
doubly-split 3-21G, 3-21Gꢃ, and 6-31 þ Gꢃ basis sets was
employed for geometry optimization as well as for
Mulliken and natural population atomic (NPA)51–54
EXPERIMENTAL
Materials
MNBS,
(C16TBuB) Hexadecyltrimethylammonium bromide
Hexadecyltributylphosphonium
bromide
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 281–290

288
M. M. MOHAREB ET AL.
Figure 7. Structures of the TMA, DMEA, DEEA, TMP, TPP, and TBP cationic head groups (with ethyl side chains) optimized at
the HF/3-21G level. Mulliken and NPA (in bold) charges with hydrogens summed to heavier atoms predicted with the HF/6-
31 þ Gꢃ method are shown
(CTAB) were obtained from Sigma-Aldrich, the alkyl-
triphenylphosphonium bromide (CnTPB) surfactants
were obtained from Lancaster Synthesis and used as
received. Hexadecyldiethylethanolammonium bromide
(C16DEEA) was synthesized28 (>99.2% pure) with
starting materials Dimethyl ethanol amine and diethyl
ethanol amine, n-bromododecane, n-bromotetradecane,
and n-bromohexadecane obtained from Sigma-Aldrich.
All surfactant solutions were prepared freshly in doubly
distilled deionized water before use.
Kinetic measurements
The reaction rates were followed spectrophotometrically
using a Hewlett Packard 8452A diode array single beam
spectrophotometer. The substrate (MNBS) was added to
the reaction cell as 25 mL of stock solution in acetonitrile
so that reaction solutions never contained more than 1%
by volume acetonitrile, and the final substrate concen-
tration was 1.0 ꢂ 10ꢀ4 M. The solution under investi-
gation without the substrate was used as the reference
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 281–290

SN2 REACTION OF A SULFONATE ESTER
289
˚
Figure 8. Structures, selected geometries (in A), and selected NPA chaꢃrges (in bold) predicted for bromide interacting with
TMA and TMP head groups (Br-TMA and Br-TMP) using the HF/6-31 þ Gꢃ method. Structures of bromide interacting with TBP
and TPP head groups, Br-TBP and Br-TPP, predicted with the HF/3-21G level are reported
solution. Due to interference from the UV absorption of
the phenyl rings in the CnTPB surfactants with respect to
MNBS absorbance, measurements for kinetic data were
taken at 290 nm for the phosphonium bromide surfactants
and for C16TBuB, absorbance was measured at 280 nm.
The temperature for all kinetic runs was maintained at
298.2 K using a thermostated Haake DC-3 water bath and
water-jacketed cell compartment.
times to show that rate constants were reproducible within
a precision of 5% or better.
CONCLUSIONS
1. Three to eight fold rate increases have been observed
for the CnTPB surfactant series for the reaction of
MNBS with bromide ion over those found for other
cationic surfactants investigated for this and other very
similar reactions. The pseudophase model shows that
the rate enhancement is due to catalysis at the micellar
surface.
Observed pseudo first-order rate constants (kobs) were
obtained from the slopes of plots of ln((A ꢀ A0)/
1
(A1 ꢀ At)) versus time, where A , At, and A0 are
1
absorbencies at the end of the reaction, at time t and at
time zero, respectively. Under the working conditions, the
pseudo first-order kinetic plots were linear for at least five
half-lives. Experiments were repeated at least two to three
2. The rate enhancement in the presence of C16TBuB is
significantly higher than found for CTAB and, given
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 281–290

290
M. M. MOHAREB ET AL.
16. Prasad M, Moulik SP, Palepu R. J. Colloid Interface Sci. 2005;
284: 658.
the relationship between observed rates for trialky-
lammonium surfactants in an analogous reaction,
suggests a possible rate enhancement due to the phos-
phorus center.
17. Jaeger DA, Bolikal D. J. Org. Chem. 1986; 51: 1352.
18. Moulik SP. Curr. Sci.. 1996; 71: 368.
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3. Mixed C14TPB-C16DEEA and C14TPB-CTAB surfac-
tant systems have shown a negative deviation from
ideality in both observed rate constants and deter-
mined km2 /Vm parameters with solution composition,
while the mixed C16TPB-C10TPB system exhibited
near ideality.
4. Mulliken and NPA analyses have shown significant
differences in charge distribution for the nitrogen- and
phosphorus-containing head groups, which may result
in any observed differences in kinetics between
ammonium and phosphonium surfactants.
22. Calvaruso G, Cavasino FP, Sbriziolo C, Liveri MLT. J. Chem. Soc.,
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5. Theoretical study at HF level has shown that for TPB,
bromide interacts only with two phenyl substituents
while for other head groups three substituents are
involved in the interaction. That allows more room
for substrate binding in CnTPB surfactants.
28. A) Glennie RA, Mohareb MM, Palepu R. J. of Dispersion Science
(Accepted for publication 2006 ). B) Mohareb MM, Ghosh KK,
Palepu RInternational J. of Chem. kinetics (In print 2006 ).
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KKG acknowledges St. FX University for the James
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